In recent years, rechargeable secondary batteries have been widely used as an energy source for wireless mobile equipment. Among other things, there has been an increasing demand for lithium secondary batteries due to various advantages such as a high-energy density, a high-discharge voltage and a superior power output stability.
Generally, the lithium secondary battery uses a metal oxide such as LiCoO2 as a cathode active material and a carbon material as an anode active material, and is fabricated by disposition of a porous polyolefin separator between the anode and the cathode and impregnation of the resulting electrode assembly with a non-aqueous electrolyte containing a lithium salt such as LiPF6. Upon charging, lithium ions deintercalate from the cathode active material and intercalate into a carbon layer of the anode. In contrast, upon discharging, lithium ions deintercalate from the carbon layer of the anode and intercalate into the cathode active material. Here, the non-aqueous electrolyte serves as a medium through which lithium ions migrate between the anode and the cathode. Such a lithium secondary battery must be basically stable in an operating voltage range of the battery and must have an ability to transfer ions at a sufficiently rapid rate.
Safety evaluation and safety securing are very important in the lithium secondary battery, and there is an urgent need for a battery with a secured safety against fire and explosion.
Meanwhile, the lithium secondary battery has advantages such as high energy density and high discharge voltage, as described above, but suffers from problems such as the risk of a momentary flow of high energy due to a high operating potential of the battery, and battery explosion or fire which may occur due to the formation of dendrites of a lithium metal on an anode surface, upon overcharge of the battery.
One of the most dangerous situations which may occur upon overcharge of a battery is “high-temperature overcharge”. When the lithium ion battery is overcharged to a level of 4.2 V or higher, the electrolyte begins to be decomposed, and a higher charge temperature leads to a higher probability of ignition because the battery temperature increases to easily reach an ignition point. However, ignition is difficult to occur in the closed space of a battery as oxygen is not supplied thereto. LiCoO2 which is used as a cathode active material in the battery has a crystal structure of “O—Co—O—Li—O—Co—O” where Li is interposed between the layered structures of “O—Co—O” in which a Co layer is positioned between oxygen atom layers. Such a crystal structure is not stable. Therefore, at a high temperature, LiCoO2 has a great tendency to be converted into a stable spinel structure. This spinel has a molecular formula of LiCo2O4, and thus has a small amount of oxygen per unit cell, as compared to the layered structure. Therefore, extra oxygen atoms dissociate from the crystal structure and migrate to the electrolyte. As a result, this leads to a supply of oxygen to the electrolyte which reached the ignition point, thereby causing explosion of the battery due to ignition.
As an approach to prevent explosion of the battery which may occur under the above-mentioned high temperature conditions or overcharged state, there has been proposed a method of mounting a protection circuit on a battery or a method of using thermal choking by a separator. However, utilization of the protection circuit requires an additional installation space inside the battery, which consequently limits miniaturization and low-cost production of a battery pack. Further, a thermal choking mechanism using the separator may not effectively work upon rapid heat generation, so it is difficult to achieve reliable operation properties.
As a scheme to solve the above-mentioned problems, there has been recently proposed use of an organic electrolyte additive. For example, Japanese Unexamined Patent Publication No. 2004-247187 and Korean Patent Application Publication No. 2003-61219 A1 disclose a technique of inhibiting an overcharge reaction, wherein addition of cyclohexyl benzene (CHB) to an electrolyte leads to a progress of an exothermic oxidation reaction under an abnormal high voltage, arising from the overcharge of the battery, and the resulting heat elevates an internal temperature of the battery within a short period of time to thereby block pores of a separator. However, due to a relatively high decomposition voltage of CHB upon addition of CHB alone, decomposition of CHB is difficult to occur when the battery is overcharged under a low voltage. Further, when the battery is left at high temperatures, oxidative decomposition of CHB leads to problems associated with the evolution of a large amount of gas and consequently swelling of the battery, thus resulting in deterioration of the battery performance.
In order to keep pace with the continuing trend towards higher functionalization and diversification of functions of a variety of electronic devices to which a secondary battery is applied, there is also an increasing need for batteries which are capable of operating under high voltage/high current conditions. However, the aforementioned electrolyte additive usually suffers from a problem associated with the poor overcharge safety under high voltage/high current conditions.
For these reasons, there is an urgent need for development of a more effective technique which is capable of securing the battery safety under high temperature conditions and upon overcharge of the battery while not causing deterioration of the battery performance, and is particularly capable of securing the battery safety even under high voltage/high current conditions.
In this connection, the present invention proposes a scheme which is capable of obtaining unexpected significant overcharge-prevention properties, particularly overcharge-prevention properties even under high voltage/high current conditions, by adding a given content of a specific combination of 2-fluoro biphenyl (2-FBP) and cyclohexyl benzene (CHB) to an electrolyte.
Even though some conventional arts suggest the use of the additive such as 2-FBP, CHB, or the like, as an electrolyte additive for the secondary battery, there is no case demonstrating the fact that use of the specific combination of 2-FBP and CHB, as will be illustrated in the present invention, can bring about unexpected significant effects on overcharge-prevention properties.
For example, Japanese Unexamined Patent Publication No. 2003-257479 discloses a non-aqueous electrolytic solution for a lithium secondary battery, comprising a fluorine-substituted aromatic compound and an aromatic hydrocarbon compound, wherein the non-aqueous electrolytic solution contains 0.1 to 20% by weight of the fluorine-substituted aromatic compound and 0.4 to 3% by weight of the aromatic hydrocarbon compound.
Japanese Unexamined Patent Publication No. 2004-134261 discloses a non-aqueous electrolyte comprising Component A and Component B as sub-solvents, wherein Component A is at least one selected from the group consisting of cyclohexyl benzene, biphenyl and diphenyl ether, and Component B is a compound having an oxidation potential higher than that of Component A, wherein the sub-solvent is added in an amount of 0.01 to 5% by weight based on the total weight of the electrolyte and the ratio of Component B in the sub-solvent is 20 to 99% by weight.
Further, Japanese Unexamined Patent Publication No. 2003-308875 discloses a nonaqueous secondary battery electrolyte comprising at least one selected from a sultone compound, cyclic sulfate and vinylene carbonate and at least one selected from a cycloalkyl benzene derivative (such as CHB) and a biphenyl derivative (such as 2-FBP).
However, the aforementioned conventional arts merely exemplify various kinds of materials that may be added to the electrolyte, and do not suggest that a specific combination of such additive compounds and a content range thereof in accordance with the present invention, as will be illustrated hereinafter, will bring about significant synergistic overcharge-prevention effects. Such a discovery in accordance with the present invention is also apparent from the fact that working examples of the aforesaid patent applications exemplify no use of such a combination.
Meanwhile, Japanese Unexamined Patent Publication No. 2002-313415 discloses a non-aqueous electrolyte comprising biphenyl and cyclohexyl benzene (CHB) as additives, wherein amounts of biphenyl and cyclohexyl benzene added to the non-aqueous electrolyte are 0.5 to 1.5% by weight and 0.5 to 2.0% by weight, respectively. However, according to the experiments performed by the present inventors, it was confirmed that the aforesaid art suffers from a significant increase in an oxidation current value and a low value in exothermic energy, so sufficient high temperature properties and overcharge-prevention properties are not achieved. In this connection, Experimental Examples, which will be illustrated hereinafter, provide analysis experimental results of overcharge characteristics and battery properties for an additive made of a combination of 2-FBP and CHB in accordance with the present invention and an additive made of a combination of biphenyl and CHB in accordance with the aforesaid conventional art.